The invention relates to a process for producing solids made from polyisocyanurate plastics, and to the solids made from polyisocyanurate plastic obtainable therefrom.
Polymers having polyisocyanurate structure are known for their high thermal and flame stability. Polyisocyanurate-containing foams based on aromatic diphenylmethane 4,4′-diisocyanate (MDI), and also polyether polyols and polyepoxides, are in widespread use especially as high-performance insulation materials, for example because of their very low thermal conductivity. See C. E. Schildknecht and I. Skeist, Polymerization Processes, pp. 665-667, Wiley, New York (1977). However, the processes for producing such foams have multiple stages and are time-consuming. The need for blending with polyols or polyepoxides because of the incompatibility of the polyisocyanurates formed in their own starting materials limits the use thereof in high-temperature applications. Moreover, MDI polyisocyanurate-containing foams, as is commonly known from aromatic polyurethanes, have only a low light stability and have a tendency to severe yellowing.
There has therefore been no lack of attempts to synthesize polyisocyanurate plastics based on aliphatic light-resistant isocyanates.
For example, European Polymer Journal, vol. 16, 147-148 (1980) describes the catalytic trimerization of monomeric 1,6-diisocyanatohexane (HDI) at 40° C. to give a clear transparent polyisocyanurate plastic free of isocyanate groups. For this purpose, however, very high catalyst concentrations of dibutyltin methoxide as trimerization catalyst are required, and these have a severe adverse effect on the thermal stability and colour stability of the products. European Polymer Journal, Vol. 16, 831-833 (1980) describes the complete trimerization of monomeric HDI to give a polyisocyanurate at a temperature of 140° C. using 6% by weight of tributyltin oxide as catalyst.
The thesis Theo Flipsen: “Design, synthesis and properties of new materials based on densely crosslinked polymers for polymer optical fiber and amplifier applications”, Rijksuniversiteit Groningen, 2000 describes the trimerization of monomeric HDI with a neodymium/crown ether complex as catalyst. The polyisocyanurate obtained, which is said to have good optical, thermal and mechanical properties, was studied in the context of the thesis for its suitability for optical applications, especially as polymeric optical fibres. Flipsen gives a detailed description of the prerequisites for clear non-yellowed polyisocyanurates. Explicit mention should be made here of avoidance of impurities, water, dimers, high catalyst concentration and high temperatures at the start of the reaction. Troublesome side reactions are reaction with water to give ureas, and of uretdiones to give carbodiimides with blister formation. According to Flipsen, only under ideal conditions with a soluble neodymium-crown ether catalyst and a preliminary reaction at 60° C. or room temperature and further reaction at temperatures of up to 140° C. are high-transparency polyisocyanurates having a glass transition temperature (Tg) of 140° C. obtained over a long period of greater than 24 h. A disadvantage of the process described is that it is a multistage process with a complicated reaction regime with problematic industrial scale implementation.
Eur. Polym. J. Vol. 18, pp. 549-553, 1982, describes a process for producing foams, wherein the trimerization is conducted of 100 g of HDI in the presence of 2% tributyltin oxide at 140° C. over 2 h with evaporative cooling up to conversions of about 50% and then cooled to 60° C. After adding 2.5% of an 8% solution of the cobalt naphthenate cocatalyst in DMSO and adding 8 g of Freon (blowing agent), the reaction commences at 60° C., exploiting the exothermicity of the reaction in order to increase the temperature of the mixture up to 200° C. within a few minutes. The mixture is subsequently kept at a constant temperature of 40° C. overnight. The conversions thus obtained vary in the region of 90%. The foams thus obtained were not examined any further for extractable components. The process described for production of polyisocyanurate foams is not implementable industrially on a large scale. The expected high HDI residual monomer content at 50% conversion and the very rapid onset reaction of the co-catalysed mixture with resulting temperatures of up to 200° C., in combination with a flashpoint of monomeric HDI of 140° C., lead to a mixture that cannot be processed safely under the typical foam production methods as a slabstock or belt foam.
The preparation of polyisocyanurates is described in the prior art mainly proceeding from liquid monomeric diisocyanates (e.g. stearyl diisocyanate, dodecyl diisocyanate, decyl diisocyanate, nonyl diisocyanate, octyl diisocyanate, HDI, BDI, PDI, IPDI, H12MDI, TDI, MDI, NDI, NBDI), of aliphatic and aromatic nature alike. The exothermicity of the trimerization reaction to give polyisocyanurates is so high (−75 kJ/mol of NCO) that a reaction proceeding from monomeric diisocyanates, particularly in the case of monomeric diisocyanates having a high isocyanate content (e.g. BDI, PDI, HDI, TIN), typically cannot be effected on the industrial scale and under adiabatic conditions as typically occur within solids in strongly exothermic polymerization processes, but only in small amounts of substance under strict temperature control.
An adiabatic change of state is a thermodynamic process in which a system is converted from one state to another without exchanging thermal energy with its environment. “Adiabatic conditions” is understood here to mean that complete dissipation of the heat of reaction released in the exothermic reaction to the environment is impossible. Thus, it is typically not possible to achieve homogeneous conditions in solids, and “adiabatic” conditions that occur particularly within the solids in the case of rapid reactions can lead to a local significant increase in temperature in the case of exothermic reaction. These local hotspots are extremely critical when the aim is the production of functionally homogeneous products.
A further problem is that aromatic monomeric diisocyanates and many arylaromatic or alicyclic monomeric diisocyanates can be homo- and co-trimerized only to low conversions. It is often necessary to add plasticizing or co-dissolving co-reactants. Otherwise, the reaction stops at high residual isocyanate contents and typically cloudy and discoloured products are obtained. The use of plasticizing and co-dissolving co-reactants is disadvantageous in turn since they lead to less chemically and thermally inert structural buildup elements such as allophanates, ureas, urethanes, thiourethanes and oxazolidinones, polyesters, polyethers, and at high temperatures to uretdiones with subsequent carbodiimidization and carbon dioxide elimination, and asymmetric isocyanurates. The production of polyisocyanurates having substantially or exclusively isocyanurate structures as structural buildup element is therefore impossible.
Temperature control in the production of polyisocyanurates having high conversion levels is of enormous significance since, because of the high isocyanate contents of the monomeric starting materials, under adiabatic conditions as typically exist in trimerizations in solids, and because of the exothermic reaction, temperatures of more than 300° C., i.e. above the flashpoint of monomeric HDI of 140° C. and the boiling point of monomeric HDI of 255° C., and even up to the self-ignition temperature of HDI of 454° C. can arise. Thus, the high temperatures can lead to direct breakdown of the products and even to in situ evaporation and self-ignition of the monomeric diisocyanates.
Aside from the occupational hygiene drawbacks resulting from the monomeric diisocyanates or breakdown products released, the formation of blisters at relatively high temperatures is very troublesome. Blisters are formed, for example, because of side reactions resulting from uretdione formation and subsequent carbodiimidization with elimination of carbon dioxide. The solids produced proceeding from monomeric diisocyanates therefore typically have blisters and thus cannot meet particular requirements relating to density, electrical insulation characteristics and mechanical properties.
Therefore, the only practical applications found by polyisocyanurates to date have typically been as crosslinking agents in paint chemistry, the preparation of which involves stopping the trimerization reaction at low conversions and removing excess unreacted monomeric diisocyanate. Thus, in the production of crosslinking agents based on monomeric isocyanurates, proceeding from aliphatic and mixed aliphatic and aromatic monomeric diisocyanates, DE 31 00 263; GB 952 931, GB 966 338; U.S. Pat. Nos. 3,211,703, 3,330,828 envisage conducting the reaction either in dilution or only up to low conversion values with very exact temperature control. There is deliberately no formation here of crosslinked polyisocyanurate plastics, but only of oligomeric, soluble products of low viscosity.
What is common to the abovementioned processes is that the trimerization is started at low temperatures. High trimerization temperatures, particularly at the start of the trimerization, can be controlled only with difficulty proceeding from monomeric diisocyanates, and lead to considerable side reactions in the form of uretdiones and carbodiimides, and are thus the cause of blister formation as a result of carbon dioxide elimination and discolouration of the product obtained. The only exception is trimerization in the presence of high concentrations of extremely slow-acting catalysts, for example tributyltin oxide. The typically multistage preliminary reactions thus conducted to give low isocyanate conversions of about 50% at temperatures above 100° C. are too costly and inconvenient for production of solids from polyisocyanurate plastic and are therefore of no interest on the industrial scale.
WO 2015/166983 discloses the use of isocyanurate polymers for encapsulating LEDs. Whereas the method of the present invention yields polyisocyanurate plastics with a good optical quality after curing times of as little as 15 minutes, the process described in WO 2015/166983 requires curing times of at least hour.
U.S. Pat. No. 6,133,397 only discloses coatings made by trimerizing oligomeric polyisocyanates. It does not disclose the production of solid bodies. What is also common to the processes described is that they are unsuitable for obtaining polyisocyanurate plastics in efficient industrial processes, particularly under adiabatic conditions as typically occur within solids in strongly exothermic reactions, especially those which are substantially free of troublesome defects in the form of discolouration, inhomogeneity and, for example, unwanted blisters. Nor is it possible in this way, by the processes known from the prior art, to effect polymerization at elevated temperatures in open reaction vessels without risking significant release of monomeric diisocyanates into the environment.
By contrast, industrially efficient processes are notable for high conversion rates and high process safety in terms of occupational hygiene.
The problem addressed by the invention was therefore that of developing an efficient industrial process for producing polyisocyanurates with a high conversion level for solids made from polyisocyanurate plastics which feature excellent weathering and chemical stability, and also high thermal stability and good mechanical properties. Ideally, these solids should lack defects such as blisters, streaks and discolouration.